(1) Field of the Invention
The invention relates to the general field of liquid crystal displays (LCDs), more particularly to methods for improving response time together with providing wider viewing angle capability, and specifically to mixing Nematic Liquid Crystal (NLC) with chiral dopant and a monomer.
(2) Description of the Prior Art
The twisted nematic (TN) and super twisted nematic (STN) are widely used in liquid crystal displays. Characteristically, however, they suffer from narrow viewing angles when applied to a large panel. The viewing angle of a display is defined in FIG. 1A. If the viewing angle had perfect characteristics the optical performance would be the same, no matter from which direction a viewer sees the display.
The NLC is a kind of uniaxial material whose refractive index can be represented by an ellipsoid having three primary axes n.sub..vertline..vertline., n.sub..perp., and n.sub..perp., as shown in FIG. 1A. The longest axis of the ellipsoid is called the optical axis. FIG. 1B demonstrates that there are two eigen-modes of light propagating in an NLC. One is the ordinary wave, with refractive index n.sub..perp., while the electric field component of the light is perpendicular to the optic axis and the direction of propagation. The other is the extraordinary wave with refractive index n.sub.eff, ranging from n.sub..perp. to n .sub..vertline..vertline. depending on the direction of the electric field, the electric field component of the light being in the same plane as the optic axis and the direction of propagation. If the angle between a light ray passing through the NLC and the optic axis is .phi., the refractive index of the extraordinary wave is EQU n.sub.eff =n.sub..vertline..vertline. n.sub..perp. /(n.vertline..vertline..sup.2 sin.sup.2 .phi.+n.sub..perp..sup.2 cos.sup.2 .phi.) .sup.1/2.
It is self-evident that n.sub.eff .gtoreq.n.sub..perp.. Because the refractive indices of the ordinary and extraordinary waves are different, there is a phase retardation .delta.(.phi.), a function of (n.sub.eff -n.sub..perp.), defined as the difference in optical path between the extraordinary and ordinary wave propagating in the NLC. Under crossed polarizers, the phase retardation strongly determines the transmittance.
FIG. 1C demonstrates that phase retardation is different when a biassed TN LCD is viewed from a different viewing angle. The bias voltage makes the liquid crystal molecules incline at a tilt angle .phi. relative to the vertical. The corresponding refractive index of the extraordinary wave is n.sub.eff (2.phi.) or n.sub..perp. while the entrance angle is -.phi. or .phi.. As mentioned above, the phase retardation .delta.(2.phi.) viewed from -.phi. and .delta.(0) viewed from .phi. will be different. As a result, the transmittance viewed from -.phi. and .phi. is also different. Furthermore, while watching the display panel along a tilt direction, the contrast will degrade and the gray scale could be inverse. The viewing angle of a conventional TN LCD is limited to +15.degree. to -30.degree. with contrast ratio greater than 10 and with gray scale inversion in the vertical viewing direction.
One approach to solving the viewing angle problem for TN LCDs is a wide viewing angle LCD with negative liquid crystal filled with an array of closed cylinders known as a gibbous lattice. Also, it has been shown in the prior art that high contrast LCDs can be obtained by using a negative NLC with pependicular boundary conditions on the glass substrates. In addition to causing symmetric alignment of NLC without a rubbing process, the closed cylinders (gibbous lattices) can also reduce the gray scale inversion.
In the gibbous lattice mode, lattice ridges of reduced height are used with the hope of reducing the light leakage induced by the NLC alignment around the ridges and of enlarging the aperture ratio of the display. However, a stabilized alignment of liquid crystals is difficult to obtain by alignment layers only and the optical response is too slow for this mode to be used in a display having full motion video. It could not cope with a LCD with wide viewing angle and high image quality.
In the course of searching the prior art, the following references were found to be of interest. Handschy et al. (U.S. Pat. No. 4,813,771 March 1989) shows electro-optic switching devices using ferroelectric liquid crystals. Bos (U.S. Pat. No. 5,410,422 April 1995) shows a greyscale LCD having a wide viewing angle. Kanemoto et al. (U.S. Pat. No. 5,243,451 September 1993) shows a LCD with cholesteric liquid crystal birefringent layers. Mazaki et al. (U.S. Pat. No. 5,491,001 February 1996) shows a method of producing a viewing angle compensator for a liquid crystal display. We also note here that one of the inventors (CK Wei) filed a related patent application on May 24 1997 (application Ser. No. 08/823,389).